This invention relates generally to the field of plastics blends or blends of polyolefin plastics with other polymers and, optionally, process aids, fillers, etc. Specifically, it is directed to blends of polyolefin plastics such as polypropylene, polypropylene reactor copolymer, polybutylene, linear low density polyethylene, and high density polyethylene with a polymeric concentrate. The polymeric concentrate is preferably elastomeric in nature and comprises a blend of polyethylene and/or a polymeric copolymer of ethylene, olefinic elastomer, oil and, optionally, process aids, antiblock additives, antioxidants, fillers, pigments, foaming agents, etc. More specifically, it is directed to compositions comprising blends of polypropylene and/or high density polyethylene and an elastomeric plasticizer. The elastomeric plasticizer is readily dispersed and the final compositions can be processed into films on equipment normally intended for low density polyethylene. Films produced from such compositions exhibit low secant modulus, high tear strength and high impact resistance compared to unmodified polyolefin plastics.
Polypropylene film is well known in the art, but it is typically produced by the cast or tubular water bath processes rather than employing the less complex equipment used for low density polyethylene. Polypropylene film produced using low density polyethylene equipment generally has low impact strength, is very stiff (high secant modulus) and has poor tear strength. Blending polypropylene with various elastomers can improve the impact strength of the polypropylene, but the overall balance of properties is jeopardized, e.g., tensile strength, tear strength, impact resistance and stiffness. More typically, ternary compositions are employed comprising polypropylene, elastomer and high density polyethylene (HDPE) wherein the desired components are directly blended, see, e.g., "Exxon Elastomers for Polyolefin Modification" (Exxon Chemical Company publication, pages 15-35). This reference suggests that the elastomer and HDPE are incorporated at levels of 5 to 15 weight percent. In addition to improved impact strength it is also stated that tensile strength and melt flow rate are depressed and, furthermore, there is no teaching that compositions prepared by merely directly blending the individual components results in a composition which can be processed on low density polyethylene film equipment.
Limited teachings are available which disclose the necessary incorporation of oil in a blend composition such as disclosed herein. However the proportions and properties disclosed in the references are quite different from what is contemplated herein. Furthermore, no reference teaches the unique method of preparing an elastomeric plasticizer which is then dispersed in the polyolefin phase, resulting in particularly desirable and useful properties in the final composition. U.S. Pat. No. 3,957,919 describes a thermoplastic elastomer composition which, optionally, can contain extender oil. The composition is hot blended in the presence of a free radical generating catalyst so as to subject the composition to a free radical reaction. The advance represented by this reference is not directed to the subject of the present invention but merely provides a means, specifically the presence of polyethylene (or polyethylene containing 10% or less of copolymerized alpha-olefins having 3 to 16 carbon atoms), in a blend of polypropylene and EPM (ethylene-propylene copolymer) or EPDM (ethylene-propylene terpolymer) which permits the use of a peroxide without significant polypropylene degradation. Furthermore, the reference specifically describes the preparation of the blend composition as a one step process; such a process is to be contrasted with the method disclosed and claimed herein which utilizes an elastomeric plasticizer concentrate which is blended with the polyolefin plastic. The types of products for which the blend composition is to be used is substantially different in the reference compared with the uses disclosed herein.
A later patent to the same inventors, U.S. Pat. No. 4,059,654, discloses the same basic composition and method as above, but the composition contains a slightly higher level of the polyethylene component so as to be better suited for use in the manufacture of flexible hose.
Elastomeric thermoplastic compositions containing oil and, optionally, a carbon black filler, for use in molded, paintable structures is disclosed in U.S. Pat. No. 4,132,698. This reference requires the polymeric components to have specific morphological characteristics in order to achieve two discrete, nonhomogeneous, continuous phases. The compositions of the reference differ from those disclosed herein with regard to the content of elastomer component as well as the contemplated uses; the instant compositions being film-forming whereas the reference is directed to molded or extruded articles. Furthermore, the concept disclosed herein of employing an elastomeric plasticizer to achieve unique blend performance properties and processability is absent from this and the other references disclosed herein.
U.S. Pat. No. 4,303,571 teaches the preparation of a film forming blend composition comprising ethylene-propylene elastomers, ethylene-vinyl acetate copolymers and a hydrocarbon oil plasticizer. However, the reference does not recognize that such a composition can be advantageously blended with polyolefin plastics nor, as taught herein, that the method of blending can be particularly beneficial.
Compositional differences also distinguish the instant invention from other references disclosing thermoplastic elastomer compositions, for example Japanese Kokai to Sumitomo Chemical, No. 77 126,450 and Japanese Kokai to Mitsui Petrochemical, No. 80 71,739. The former reference discloses a modified copolymer rubber composition for use in extruded and molded articles. Additionally, the compositions of the latter reference are intended to be treated with a peroxide crosslinking agent.